An x-ray source is often used is medical imaging systems such as but not limited to, computed tomography, fluoroscopy and mammography systems. The x-ray source typically includes an evacuated vessel known as a frame comprising a cathode and an anode. X-rays are produced by applying a high voltage across an anode and a cathode, and accelerating electrons from the cathode towards a focal spot on the anode.
Cathode assemblies for such x-ray sources typically include a cathode cup and a plurality of current carrying filaments. The filament leads extend through the cup via the filament feed-through assembly, which typically comprises an electrical insulator and a metallic sleeve used for securing the leads at the desired location. The filaments are energized so that electrons accelerate towards the anode to form focal spots. Current cathode cups have shallow channels where the filaments are placed. Further in the absence of external magnetic fields or any biasing electric fields, focal spot size is mainly dependent on filament coil diameter and cathode cup geometry. Therefore it is not easy to achieve focal spot size below 0.3 nominal. The focal spot size is also very sensitive to filament set height which affects the manufacturing cost and degrades the yield of the manufacturing process. Filament set height is the height from the cathode cup channel surface to the tip of the filament positioned in the cathode cup. Further the focal spot size will be also dependent on a radius of the filament channel edge or chamfer. Any change in the radius can lead to a more costly cathode production and/or focal spot size failures during operation. Present methodologies of producing small focal spots require presence of external magnetic or biasing electric fields and/or variation in the filament coil diameter which affects the manufacturing cost.
Accordingly, a need exists for an improved system for achieving smaller focal spot size in an X-ray tube in an efficient manner.